Semiconductor Light Emitting Diodes, commonly referred to as LEDs, were introduced in the 1960's when visible red light was produced using gallium arsenide phosphide (GaAsP) on a GaAs substrate (Ref. N. Holonyak Jr. and S. F. Bevacqua, “Coherent (Visible) Light Emission from Ga(As1−xPx) Junctions,” Appl. Phys. Lett., vol. 1, pp. 82-83, 1962.). Over the last four decades significant improvements in LED technology, availability of other semiconductor materials, and, generally, optoelectronic technology have led to more efficient devices being produced over a wider spectrum of visible color.
The illumination produced by LEDs is generated by radiative recombination of electrons and holes in a semiconductor device, generating light (photons) through the process of electroluminescence. In doped semiconductor material, impurity atoms change the electron balance, either adding free electrons or creating holes where electrons can migrate. Either of these additions makes the material more conductive. A semiconductor with extra electrons in the conduction band is called n-type material; free electrons move in the conduction energy band through the processes of diffusion and drift. A semiconductor with extra holes is called p-type material, since it has extra valence electron deficiencies (holes); the holes move in the valence energy band as positive charges through the processes of diffusion and drift. A heterostructure LED comprises a section of n-type material and a section of p-type material with an active layer in between, sometimes quantum sized, and with electrodes disposed in electrical communication with the n and p sections.
Light is produced by double heterostructure and “quantum well” LEDs when free electrons from the n-type layer recombine in the active layer with holes from the p-type layer. For every electron that falls from the conduction band to the valence band, there is a possibility of producing one photon, resulting in the illumination. The probability that a photon will be produced by recombination of a given electron is the internal quantum efficiency of the material. Visible light is only produced when the diode is composed of certain materials, so called “wide bandgap” materials, with a direct energy gap in the range of visible light. Until recently, it was not possible to use LEDs for general lighting applications, because general “white” lighting requires a blending of photons with several different energies, e.g. red, green, and blue, and the technology did not exist to make bright blue emitters.
Modern innovations in LED technology have led to the use of III-V semiconductor materials to produce high-efficiency LEDs at both ends of the visible spectrum. For example, III-arsenide-phosphide (III-AsP) materials have been used since the 1960s to produce yellow to infrared LEDs, and III-nitride (III-N) materials have been used since the mid-1990s to produce blue-green to ultraviolet LEDs. [Ref: Shuji Nakamura and Gerhard Fasol, The Blue Laser Diode, Springer, Berlin (1997)] The most efficient LEDs are made from double heterostructures, with an extremely thin “quantum sized” layer of light emitting alloy sandwiched between larger-bandgap and thicker p-type and n-type layers. The active layer in such devices is commonly referred to as the “quantum well” and is strictly defined as a one-dimensional (ID) potential well for electrons and holes whose width is of order the same or thinner than the free-exciton Bohr radius. In a true quantum well, electrons from the n-type layer and holes from the p-type layer exhibit ID confinement, being localized in the quantum dimension, and forming essentially 2-dimensional (2D) wavefunctions in the quantum well.
III-AsP device heterostructures are typically grown epitaxially on high quality bulk III-V substrates (e.g. GaAs) and the crystal quality in the active layers is very good, with on the order of 1000 crystal dislocations per square centimeter (cm) or less. As such, in III-AsP devices the electron-hole wavefunctions are truly 1D confined, as previously discussed. FIG. 1 provides a cross-sectional view of a III-AsP optoelectronic device, in accordance with the prior art. The device 10 includes a substrate 12 that is formed of gallium arsenide (GaAs), a n-type conductive layer 14 that is formed of n-type aluminum indium gallium phosphide (AlInGaP), a quantum well or active layer 16 that is formed of indium gallium phosphide (InGaP) and a p-type conductive layer 18 that is formed of p-type AlInGaP. This device is exemplary of a red LED heterostructure with ID confinement.
III-Nitride (III-N) device heterostructures are typically grown on sapphire or silicon carbide (SiC) substrates. Due to lattice and thermal conductivity mismatch between the substrate and the III-Nitride, the crystal structure in the active layer is low quality, exhibiting up to 109 dislocations per square cm. FIG. 2 provides a cross-sectional view of a III-N optoelectronic device, in accordance with the prior art. The device 20 includes substrate 22 that is formed of sapphire, n-type conductive layer 24 that is formed of n-type gallium nitride (GaN), quantum well or active layer 26 that is formed of indium gallium nitride (InGaN) and includes high-indium-fraction InGaN quantum dots 28, and p-type conductive layer 30 that is formed of p-type GaN. Quantum-sized indium-rich dots, or nanoparticles, form spontaneously in InGaN quantum well layers grown with metal-organic chemical vapor deposition (MOCVD). [Ref: K. P. O'Donnell, R. W. Martin, and P. G. Middleton, Phys Rev Let, 82, 237 (1999)] Since the solubility of indium in InGaN is a function of the thermodynamic state of the InGaN, quantum dot formation in InGaN active layers is driven by the variations in thermodynamic state during the MOCVD process. The quantum dot formation process can now be empirically controlled to yield efficient light emitting diode wavelengths over a range from about 380 nanometers (nm) to about 520 nm. For wavelengths shorter than 380 nm, the relative lack of indium in the InGaN alloy does not allow for efficient light emitting devices and beyond 520 nm the InGaN nanostructure does not result in efficient device performance.
In the III-Nitride optoelectronic device illustrated in FIG. 2, if the electron-hole wavefunctions were simply confined in the quantum well as in III-AsP devices, then the III-N devices would not be very efficient because the 2D electron and hole wavefunctions would simultaneously intersect all the crystal dislocations 32 that act as non-radiative electron-hole recombination centers (NRRC) for electrons and holes. However, the InGaN active layer 26 exhibits strong InN—GaN material segregation because the layer consists of high-indium fraction InGaN quantum dots 28 in a low-indium fraction InGaN quantum well. Electron-hole pairs are confined in three-dimensions to the smaller bandgap higher-indium InGaN quantum dots and thus do not interact with the crystal dislocations. Both compositional and quantum-size effects provide for the quantum dots to illuminate visible blue light. Thus, it is possible to make high efficiency blue InGaN LED devices in spite of the high crystal dislocation densities.
FIG. 3 provides a cross-sectional view of a III-N optoelectronic device that exhibits epitaxial lateral overgrowth (ELOG), in accordance with the prior art. The ELOG process results in significantly lower threading dislocation density. The device 40 includes a substrate 42 that is formed of sapphire, n-type conductive layer 44 that is formed of n-type gallium nitride (GaN) and includes rows of silicon dioxide (SiO2) stripes 46. The stripes serve to stop the dislocations 48 emanating from the substrate before they propagate into the active layer. Thus, the stripes tend to filter the dislocations and inhibit epitaxial lateral overgrowth of the conductive layer. The device additionally includes quantum well or active layer 50 that is formed of indium gallium nitride (InGaN) and includes high-indium-fraction InGaN quantum dots 52 and p-type conductive layer 54 that is formed of p-type GaN. This structure yields an even more efficient blue InGaN LED device then the example provided in FIG. 2.
While III-AsP and III-Nitride are good materials for high-efficiency red and blue LEDs and laser diodes, neither provides for high-efficiency deep green devices; i.e. devices that operate in the 555-585 nanometer range near the peak of the human eye response curve. [Ref: FIG. 6 in A. Bergh, G. Craford, A. Duggal, and R. Haitz, Physics Today, December 2001, p. 54] In this spectral region, recently commercialized cadmium selenide (CdSe) quantum dots may provide some illumination entitlement. Recently, significant developments have been made in the deposition of thin layers of CdSe quantum dots onto solid surfaces, assembly of the dots into 3-dimensional “quantum dot solids” and incorporation into prototype microelectronic devices. For example, CdSe nanoparticles dispersed in a polymer host matrix have been used as a downconverting layer over a blue or ultraviolet LED, see U.S. Pat. No. 6,501,091, entitled “Quantum Dot White and Colored Light Emitting Diodes”, issued in the name of inventors Bawendi et al., on Dec. 31, 2002. Such quantum dot phosphor dispersions have the property of low optical scattering, since their size is significantly smaller than the wavelength of light. CdSe quantum dots have also been shown to be dispersible in an inorganic matrix. See, for example, published U.S. patent application Ser. No. 2003/0142944, published in the name of inventors Sundar et al., on Jul. 31, 2003. In addition, monolayers of CdSe quantum dots have been used as the active layer of organic LEDs with a 25 percent improvement over previous QD-LED performance and external quantum efficiency of 0.4 percent. [Ref: Coe et al, Nature, 420, 800 (2002)]
Hence, a need exists to develop and manufacture optoelectronic devices, such as LEDs, laser diodes and photodetectors that operate efficiently. In addition, a need exists to extend the wavelengths of light emitting diodes into the “deep green” range wavelengths near the peak of the human eye response curve, i.e. about 555 nm to about 585 nm. Such devices and the corresponding methods for producing such devices should be cost-effective and reliable. In addition, the desired devices should accommodate non-lattice matched substrates without having dislocations in the substrates adversely affect the performance of the devices.